A flexible polyurethane foam is produced by blending a polyurethane raw material containing a polyol, a polyisocyanate, a blowing agent, and a catalyst with an inorganic compound hydrate having a specific gravity of 1.5 to 4.0 and reacting the above polyurethane raw material to effect foaming and curing. The inorganic compound hydrate is preferably an iron sulfate hydrate, a calcium sulfate hydrate, or a magnesium sulfate hydrate. The above inorganic compound hydrate preferably has decomposition temperature of from 100 to 170.degree. C. Then, an increase in exothermic temperature owing to the above foaming and curing is diminished by evaporation of water formed through decomposition of the above inorganic compound hydrate.